Air-coupled ultrasonic is a non-contact technique which has
become increasingly common for non-destructive testing, as more and more advanced
materials cannot be contaminated during certain manufacturing processes by the various
couplants used in regular ultrasonic testing

The air-coupled ultrasonic technique has in addition shown to
be very efficient and fast for the testing of large areas, where the application of plate
waves can cover long distances, and where the absence of water columns allows for high
scan velocities.

THE OBSTACLES

The main reason for the use of a couplant lies in the large
differences in impedances between air and any solid material: The impedance for air is in
the order of 100 Rayl, whereas liquids and solids have impedances in the MegaRayl range.
Couplants are used to reduce this impedance mismatch. Without a couplant, i.e. with air as
the natural couplant, the impedance mismatch causes the following
typical high reflection losses:

Specimen : 60 - 90 dB

Transducers: 90 dB

Total Reflection Losses: 180 dB.

Another restriction to be considered
for the application of air-coupled ultrasound is the sound attenuation in air.The
following graph was calculated based on values found in: P.E. Krasnushkin, Phys Rev
190-193, 1944.

The attenuation of sound
traveling through air increases rapidly for frequencies above ~ 1 MHz.

SOLUTIONS

There are several techniques available today to overcome
above mentioned obstacles, and to make air-coupled ultrasound an efficient and reliable
method for the non-destructive testing industry. These techniques involve the following
routes:

(a) High sound pressures can compensate for
the high reflection losses.This can be obtained by using high voltage transmitters and by
applying tone bursts to resonant transducers.

(b) High sensitivity of the receiving
transducer, combined with low-noise preamplifiers. The newest tools in this category are
the digital filters, which can significantly increase the signal to noise ratio of weak
signals.

(c) Impedance matching layers for the
coupling of the transducers to air. New materials have reduced these impedance mismatch
losses, leading to a higher sound pressure in air and to a higher sensitivity.

(d) Optimizing the frequency for the
specific application.

TRANSDUCER CONFIGURATIONS

It is a characteristic of air-coupled ultrasound, that plate
waves can travel longer distances, without being dampened by a couplant.This allows to use
several transducer configurations with interesting results.

TWO-SIDED INSPECTION

Through-Transmission

Shear Wave

Plate Wave

ONE-SIDED INSPECTION

Pseudo Pulse-Echo with Sound
Barrier

Plate Wave

Some results and additional
information can be found in the following
publications:

The air-coupledultrasonic technique has become a
reliable and indispensable method in nondestructive testing. The availability of various
frequencies and digital filters allows an optimization between high resolution for
composites and honeycombs (e.g. 0.5 to 1 MHz) and high penetration for foam, pultruded
materials, rubber, tires, and wood (20 to 200 kHz).